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There are many factors that influence when a child reaches certain developmental milestones. Use this timeline to know what to expect in the first year.

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The American Academy of Pediatrics (AAP) recently issued a revised recommendation stating that children should receive two doses of the varicella, or chickenpox, vaccine. In addition to the standard first shot, given between 12 and 15 months, the AAP is advising a new booster shot, between 4 and 6 years of age, to prevent "breakthrough" infections.

For parents, this has caused some confusion: If the vaccine is effective, then why have there been breakthrough cases? And why do children need a booster shot? For that matter, some ask, why vaccinate at all—isn't chickenpox a common, and harmless, childhood illness?

Common, yes. Harmless, no. Actually, experts say, the varicella vaccine has done exactly what it was designed to do: eliminate serious chickenpox infections in children. Before the vaccine was introduced in 1995, the nearly 4 million cases annually in this country resulted in 11,000 hospitalizations and about 100 deaths.

The varicella vaccine is effective about 85 percent to 90 percent of the time, which means that approximately one in 10 children who are immunized may develop a breakthrough infection after coming into contact with someone who has chickenpox. These breakthrough cases are generally very mild, typically lasting only a few days, with little or no fever and just a few skin lesions. However, in about 30 percent of cases, kids develop more than 250 skin lesions, meaning it is possible to have a "typical" case of chickenpox after being immunized.

Increasingly, breakthrough cases have been occurring in kids around 9 to 11 years old—of particular concern, since chickenpox is most serious in adolescence and adulthood. (Note: Pregnant women should not receive the chickenpox vaccine; see "A Shot in the Arm") That's one reason for the new two-dose immunization schedule, which the AAP anticipates will increase the vaccine's effectiveness to about 98 percent and thus decrease the incidence of potential complications.

Still, many anti-vaccine proponents stress that children can receive all the benefits of lifelong immunity without a shot: by contracting chickenpox naturally. As a result, some parents forgo the vaccine in favor of "chickenpox parties," where kids are invited over to play, eat, have fun—and be exposed to a child with the disease.

Many experts counter that these parents underestimate the potentially serious complications of such an approach. "There's a perception that chickenpox is a mild disease that only causes itching," says David Kimberlin, M.D., associate professor of pediatrics in the Division of Pediatric Infectious Diseases at the University of Alabama, Birmingham. But he notes that potential side effects include bacterial skin infections, pneumonia and, rarely, encephalitis or even death.

It's not yet known whether the two-dose schedule will offer lifelong immunity. In the meantime, parents should discuss options with their pediatrician to decide which approach is best for their child.